Disc drive data storage devices of the type in which the present invention is particularly useful are well known in the industry. Such disc drives typically have a base housing to which other components are mounted. These other components consist of a spindle motor, usually of the brushless DC type, having a hub on which one or more discs are mounted for rotation at a constant speed of 3600 RPM or greater. Each disc surface contains a large number of circular, concentric data tracks onto which data can be written and from which data can be read.
The mechanism for performing these reading and writing functions is a number of read/write heads-- usually one per disc surface--that are carried from track to track by an actuator mechanism.
Actuator mechanisms generally fall into two categories:
1) linear actuators which move the heads in a straight line along a radius of the disc (or parallel to a radius of the disc), using ball bearings or self-lubricating bushings in cooperation with an arrangement of guide rails or rods, or;
2) rotary actuators which pivot the heads in an arc across the disc surface about an axis of rotation closely adjacent the outer diameter of the discs.
Both of these types of actuator require some sort of motor to provide the desired movement, and again there are two common types of motor most frequently used in the industry:
1) stepper motors, and;
2) voice coil motors (VCMs).
Both of these types of motor operate under the control of electronic circuitry to controllably move the actuator--and thus the read/write heads--in response to commands issued by a host computer or an "on-board" controller within the disc drive.
Whichever type of actuator and actuator motor are used, the allowable range of motion must be limited to ensure that the read/write heads act properly in cooperation with the discs. Therefore, both an inner limit--toward the center of the disc--and an outer limit--toward the outside diamter of the disc--must be established. It is easy to imagine the damage that could occur if the read/write heads were to move in an uncontrolled manner toward the center of the disc and collide with the hub mounting the disc. Similar damage could occur from uncontrolled movement of the heads in the opposite direction.
If perfect, fault-free operation of the controlling electronic circuitry were possible, then these limits could be established using only the electronic circuitry. However, because of the possibility of failure in components of this type, mechanical restricting means are usually employed.
These mechanical means for restricting the range of motion of the actuator and read/write heads are frequently referred to in the industry as "crash stops", and have taken many forms over the years. An example of crash stops in a linear stepper motor disc drive is disclosed in U.S. Pat. No. 4,471,396, issued Sep. 11, 1984, assigned to the assignee of the present invention and incorporated herein by reference.
Furthermore, since disc drives using VCM actuators have no inherent magnetic detent to maintain their position without power, this type of disc drive incorporates a "park position"--usually at one end of the range of motion of the actuator--to which the actuator is moved at a power loss, and some sort of latching mechanism to hold the actuator in the park position until power is restored.
Many types of latching mechanisms have been used for this purpose and examples of such latches can be found in U.S. Pat. Nos. 4,725,907, issued Feb. 16, 1988and 4,716,480, issued Dec. 29, 1987, both assigned to the assignees of the present invention and incorporated herein by reference.
In some disc drive designs the latching mechanism for holding the actuator in the park position has been integrated with the associated crash stop, as in U.S. Pat. No. 4,890,176, issued Dec. 19, 1989, and U.S. Pat. No. 5,187,627 filed Nov. 9, 1990, both again assigned to the assignee of the present invention and incorporated herein by reference.
Current state-of-the-art disc drives are capable of moving the read/write heads at speeds of 60 inches per second or more, and have track densities (measured radially on the disc surface) of 1750 tracks per inch or greater.
From these figures, it is evident that an effective crash stop must be capable of absorbing a relatively large amount of energy if the actuator reaches its "end of travel" in an uncontrolled manner. The specific amount of energy to be absorbed is dependent on the moving mass and the velocity at the time of impact. Therefore, a crash stop with an easily selected and controllable ability to absorb this type of shock is to be desired.
Furthermore, the location of the limit of travel should be easily and precisely adjustable, since small variations in this setting can involve the loss or retention of a number of data tracks, thus greatly influencing the total storage capacity of the disc drive.